(1) Field of the Invention
This invention relates to a PCM coder-decoder, and more particularly to an audio PCM coder-decoder (or PCM CODEC) for use in subscriber's circuits of a digital switchboard, or the like.
(2) Description of the Prior Art
Development of digital switchboards has been made in recent years, and a conference telephone system which realizes simultaneous exchange of telephone communication between three or more parties has been proposed as one of the applications of the digital switchboards.
In order to accomplish the conference telephone system, a part of the reception signal must be folded back and be superposed on a transmission signal in a subscriber's circuit.
The conference telephone system proposed in the past employs a method which disposes separately a PCM multiplextor besides the PCM codec, converts the multiplexed PCM signal to an analog signal and adds it, in order to realize the function described above. Therefore, this system involves the following problems.
(1) The PCM multiplexor is indispensable, and it is difficult to reduce the cost of the apparatus and to improve actual packaging density.
(2) A signal-to-noise ratio S/N deteriorates due to quantizing noise generated in the conversion process of PCM signal.fwdarw.analog signal.fwdarw.PCM signal, and the like.
(3) The operation speed of the A/D convertor becomes drastically higher in proportion to the multiplexing rate, and the design and actual packaging of analog circuits become difficult.
(4) The absolute volume of delay time becomes great for the timing adjustment in the PCM multiplexor.